Do this. It is cool.

Get some money. $20 should do. Go to radio shack. Buy five 10 kiloohm resistors, one 1000 microfarad capacitor, a 90 decibel peizo buzzer with a 5 miliamp max rating, a push button switch, and some wire.

Next, get an old electronic device that used 9 volt batteries, and remove the battery plug in. Get plenty of wire to work with. Strip the insulation off the ends of the wire. Attach the red wire to one end of the switch.

Now, take the resistors and connect them in paralell. Just twist the wires together so that the resistors are bunched up in the middle with wire on each end. Attach one end of the resistors to the red wire on the piezo buzzer. Attach the other end to the unoccupied side of the switch.

Now, attach one end of the capacitor to the switch side of the resistors.

Finally, tie the other capacitor wire and the black wire on the peizo buzzer to the black wire on the battery hook up. Install a 9V battery, and hit the switch. If it worked, then the buzzer should be killing your ears. When you turn off the switch, it should still be killing your ears.

NOTE: You do not need five resistors in series. You could use one resistor, if it had the right resistance. Remember that for resistors in series, the reciprocal total resistance is the sum of the recipriocals of the resistance of each individual resistor.

Also, remember that the colors of the wires is very important, piezo electric crystals only work if the current flows in the right direction, otherwise, there will be no sound.

For added fun, put the circuit into a box, with only the button visible. Trick people into pushing the button, then accuse them of breaking the device when it keeps going after they turn it off.

If I get around to it, I'll post a circuit diagram of my evil invention. But probably not.